The World Health Organization (WHO) has warned that the mosquito-borne Zika virus, which causes severe birth defects, is likely to spread to parts of Europe, including the Balkans and parts of the Black Sea coast.

The report, released on May 18, will deepen concern about the dangers of the virus, which has been detected in more than 50 countries. The virus has been linked to thousands of cases of a rare birth defect that causes babies to be born with an unusually small head.

The UN agency said the highest-risk regions were Portugal's Atlantic island of Madeira and the Black Sea coasts of Russia and Georgia, including the breakaway region of Abkhazia. That's because one species of mosquito that is responsible for carrying most of the infections is indigenous in those areas.

The likelihood of an outbreak is considered moderate in 18 countries where a different mosquito species breeds, the WHO said. That includes most of the Balkans, along with Turkey and Greece, the agency said.

"We call particularly on countries at higher risk to strengthen their national capacities and prioritize the activities that will prevent a large Zika outbreak," Zsuzsanna Jakab, WHO regional director for Europe, said in a statement.

In February, the agency declared the virus's spread to be a global emergency. In addition to causing severe birth defects, it is known to cause a rare neurological condition in adults that is sometimes fatal or causes temporary paralysis.

In addition to being mosquito-borne, the virus is also able to be transmitted through sexual contact.

Joao Pires, one of the authors of the report, told RFE/RL that travelers heading to high-risk regions should take precautions like wearing long-sleeved clothing and using insect repellent.

"That is what we are advising travelers: when going to countries and territories with an ongoing transmission, they have to prevent from getting bitten by mosquitoes," he said. "That is the best strategy to prevent the Zika virus and any potential complications."